Known in the art is a commercial process for purification of the off-gases resulting from the production of benzoyl chloride prepared by phosgenation of benzoic acid from phosgene. This process comprises passing the off-gases successively through two packed columns, whereinto water is fed counter-currently to the off-gases at a temperature of from 70.degree. to 85.degree. C. while into the other column an aqueous ammonia solution is fed at the same temperature. The content of phosgene in gases after said purification does not exceed 0.5 mg/m.sup.3.
This prior art process has a disadvantage residing in decomposition of phosgene which excludes recycling thereof to the production of benzoyl chloride. Besides, utilization of hydrogen chloride from the resulting diluted aqueous solution is complicated, which hinders the production of commercial hydrochloric acid and increases the amount of waste waters.
Also known are processes for purification of the off-gases resulting from other kinds of manufacture from phosgene, e.g. from the production of chloroformates or alkylchlorides with the use of phosgene such as hydrogenation (cf. USSR Inventor's Certificate No. 181621, Cl. B 01 d 53/04), passing the off-gases through a bed of activated alumina at a temperature within the range of from 110.degree. to 200.degree. C. (cf. U.S. Pat. No. 3,789,580 Cl. B 01 d 53/04) or through a bed of aluminium at a temperature within the range of from 300.degree. to 600.degree. C. (cf. FRG Pat. No. 2,115,859 Cl. C 01 d 7/58), washing of the off-gases with an alkali solution with the addition of minor amounts of ammonia (cf. FRG Pat. No. 2,531,545 Cl. B 01 d 53/02), treatment of the off-gases with ethanol at a temperature within the range of from 0.degree. to 100.degree. C. under atmospheric or elevated atmospheric pressure (cf. U.S. Pat. No. 3,142,535 Cl. 23-154).
The above-described purification processes exclude recycling to the production process, e.g. of chloroformates or alkylchlorides, since decomposition of phosgene occurs as a result of purification.
Also known in the art is a process for purification of the off-gases resulting from the production of isocyanates from phosgene, wherein the off-gases containing phosgene and hydrogen chloride are cooled to a temperature below the boiling temperature of phosgene (+8.2.degree. C.) but above the boiling point of hydrogen chloride (-85.1.degree. C.) and passed counter-currently through a bed of liquid phosgene (cf. French Pat. No. 1,089,476 Cl. C 07 C).
The use of liquid phosgene as an absorbent for recovery of phosgene from the off-gases has certain disadvantages:
1. Phosgene has a high vapour tension even at low temperatures. For this reason, prior to absorption of phosgene it is necessary to preliminary cool the off-gases. Despite the cooling, during absorption a substantial entrainment of the absorbent per se and the up-gas phosgene is possible along with inert gases. In addition, at a temperature below 0.degree. C. clogging of pipelines is possible along with elevation of pressure in the system due to congelation of benzoyl chloride. All this results in a complicated technology of purification of the off-gases.
2. Due to a high toxicity of phosgene, special safety measures are required. The use of phosgene as an absorbent impairs labour conditions and results in pollution of the environments.